WordNet sense 1 (become less tense, less formal, or less restrained, and assume a friendlier manner):

relax

WordNet sense 2 (cause to become unblocked):

unstuff

WordNet sense 3 (make one's body limber or suppler by stretching, as if to prepare for strenuous physical activity):

limber up, warm up

WordNet sense 4 (become less tense, rest, or take one's ease):

decompress, unbend, unwind

WordNet sense 5 (cause to feel relaxed):

unlax, unstrain, make relaxed